What is the original meaning of P and V operations in a context of a semaphore?

V stands for signal and P stands for wait.

You can check the wiki for details.

Why they are not called wait and signal?

I would agree with Bill on this, the most likely reason is that because there are a lot of people who dont use English, so its probably because of that it is not called wait and signal.

Dijkstra, one of the inventors of semaphores, used P and V. The letters come from the Dutch words Probeer (try) and Verhoog (increment).

See also: https://cs.nyu.edu/~yap/classes/os/resources/origin_of_PV.html

@Kris answer is just partially right.

V stands for 'Verhoog' and P stands for 'Prolaag', not 'Probeer' as cited here.

Verhoog can be translated as 'increasing'. Decreasing would be 'Verlaag', but for better distinction between the letters Dijkstra invented the word 'Prolaag'.

See the naming in the original paper.